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  • Irrigation Water

    L-5417 03/02

    Salinity is becoming a problem in many areas of

    Texas. As water quality and cropping patterns

    change, salinity may injure crops and reduce yield.

    Susceptibility to salt injury varies by crop. It is

    important that producers understand why and how

    to measure salts and how crop susceptibility

    to salts may differ.

    Why well water can be salty

    Irrigation water quality isdetermined by the totalamounts of salts and the types of salts present in the water. A salt is a combina- tion of two elements or ions. One has a positive charge (for example, sodium), and the other has a negative charge (such as chloride). Water may contain a vari- ety of salts including sodi- um chloride (table salt), sodium sulfate, calcium chloride, calcium sulfate (gypsum), magnesium chlo- ride, etc. The types and amounts of salts in water, and thus the salinity of that water, depend on the source.

    The quality of well water depends on the composi- tion of the underground formations from which the water is pumped. When these are “marine” (ocean) formations, they usually will have higher salt levels

    and produce water that is more salty. The quality of surface water depends large- ly on the source of runoff. Drainage water from

    irrigated land, saline seeps, oil fields, and city and industrial wastewaters gen- erally has higher salt levels.

    What problems can salty water cause?

    Salty irrigation watercan cause two majorproblems in crop pro- duction—salinity hazard,

    Mark McFarland, Robert Lemon and Charles Stichler*

    Critical Salt Levels for Peanuts, Cotton, Corn and Grain Sorghum

    *Associate Professor and Extension Soil Fertility Specialist; Associate Professor and Extension Agronomist; Associate Professor and Extension Agronomist

  • Table 1 Critical Values for Salts in Irrigation Water for Major Crops MEASUREMENT PEANUTS CORN GRAIN SORGHUM COTTON

    Sodium Adsorption Ratio (SAR) No units (just a number) 10 10 10 10

    Total Dissolved Salts (Electrical Conductivity or Total Dissolved Solids*) Micromhos per centimeter (umhos.cm) 2100 1100 1700 5100 Microsiemens per centimeter (uS/cm) 2100 1100 1700 5100 Millimhos per meter (mmhos/cm) 2.1 1.1 1.7 5.1 Decisiemens per meter (dS/m) 2.1 1.1 1.7 5.1 Parts per million (ppm) 1344 704 1088 3264 Milligrams per liter (mg/L) 1344 704 1088 3264

    Toxic Ions (Resulting in Foliar Injury)

    Boron Parts per million (ppm) 0.75 2.0 3.0 3.0 Milligrams per liter (mg/L) 0.075 2.0 3.0 3.0 Milliequivalents per liter (meq/L) 0.075 0.2 0.3 0.3

    Chloride Parts per million (ppm) 400-500 533 710 710 Milligrams per liter (mg/L) 400-500 533 710 710 Milliequivalents per liter (meq/L) 11-14 15 20 20

    Sodium Parts per million (ppm) 400-500 533 710 710 Milligrams per liter (mg/L) 400-500 533 710 710 Milliequivalents per liter (meq/L) 17-21 23 31 31

    *Different units of measurement for total soluble salts represent the same critical value

    and sodium hazard. When irrigation water is used by plants or evaporates from the soil surface, salts con- tained in the water are left behind and can accumulate in the soil. These salts cre- ate a salinity hazard

    because they compete with plants for water. Even if a saline soil is water saturat- ed, plant roots may be unable to absorb the water, and plants will show signs of drought stress. Foliar applications of salty water

    often cause marginal leaf burn and, in severe cases, can lead to defoliation and significant yield loss. Sodium hazard is caused by high levels of sodium, which can be toxic to plants and damage medi-

  • um and fine-textured soils. When the sodium level in a soil becomes high, the soil will lose its structure, become dense and form hard crusts on the surface.

    What tests should be done on irrigation water?

    To evaluate a salt haz-ard, a water sampleshould be analyzed for three major factors:

    • Total soluble salts.

    • Sodium hazard (SAR).

    • Toxic ions.

    Total soluble salts meas- ures the salinity hazard by estimating the combined effects of all the different salts that may be in the water. It is measured as the electrical conductivity (EC) of the water. Salty water carries an electrical current better than pure water, and EC rises as the amount of salt increases. Many people make the mistake of testing only for chlorides, but chlorides are only one part of the salts and do not determine the entire prob- lem.

    Sodium hazard is based on a calculation of the sodium adsorption ratio (SAR). This measurement determines if sodium levels are high enough to damage the soil or if the concentration is great enough to reduce plant growth. Sometimes a factor called the exchange- able sodium percentage (ESP) may be listed or dis-

    cussed on a water test; however, this is actually a measurement of soil salini- ty, not water quality.

    Toxic ions include ele- ments like chloride, sulfate, sodium and boron. Sometimes, even though the salt level is not exces- sive, one or more of these elements may become toxic to plants. Many plants are particularly sensitive to boron. In general, it is best to request a water analysis that lists the concentra- tions of all major cations (calcium , magnesium, sodium, potassium) and anions (chloride, sulfate, nitrate, boron) so that the levels of all elements can be evaluated.

    What are the critical levels?

    Agricultural crops dif-fer greatly in theirability to tolerate salts. Some crops have spe- cial methods for managing high salt levels inside the plant that allow them to continue to grow and pro- duce. In most cases, critical levels have been estab- lished for each crop and

    each type of salt test or problem. One of the most confusing factors is that there can be many different units of measurement for the same test. That is, the numbers have the same relative meaning, but the units of measurement used to express the value are differ- ent (much like saying 12 inches or 1 foot).

    The Texas Cooperative Extension Soil, Water and Forage Testing Laboratory uses standard units of micromhos per centimeter (umhos/cm) for total solu- ble salts and parts per mil- lion (ppm) for individual ions. Other laboratories may use different units of measure that can be calcu- lated by making simple conversions. Table 1 lists the different tests and cor- responding critical values for different units of meas- urement. These values rep- resent the maximum salt level in irrigation water that can be used without reducing crop yield. Keep in mind that these values are estimates. Actual crop response may vary depend- ing on soil type, rainfall, irrigation frequency and weather conditions. Note cotton’s ability to tolerate higher levels of salt than other common Texas crops.

    Management factors

    Irrigation water with asalt level near the criti-cal value is referred to as “marginal” quality water. In some cases, marginal quality water can be used

  • Water analyses can be accurate only if the sample is taken correctly. Please use the following guidelines when collecting

    a well water sample for irrigation water quality analysis:

    Containers

    Samples should be collected in a clean, plastic bottle with a screw cap.Wash bottles thoroughly before taking samples to eliminate any contami-nation. An 8-ounce plastic, disposable baby bottle is the best kind of container to use. Rinse the container several times with the water to be tested before collecting the final sample. Always clearly identify each container with a specific sample identification (well site). When mailing samples, place the bottles in a box or pack them with a soft packing material (newspaper or sty- rofoam) to prevent crushing.

    Collecting the water sample When testing well water, allow the pump to operate for at least 20 minutes before taking the sample to be sure the water is representative of what is being tested. Take the water sample at the pump so that residues from the lines do not contaminate the sample. If two or more wells supply an irrigation system, one sample may be taken from the system after pumping (flushing) for at least one hour. However, if a water test indicates a problem, all wells sup- plying the system will need to be tested individually to determine the source of the problem. Sometimes one poor quality well can dramatically reduce the quality of a mixture.

    Testing should also be done on irrigation water from ponds, reservoirs, streams or other surface water sources. Samples can be obtained by collecting water from a faucet near the pumping station after operating for 20 minutes or longer. For irrigation water sources where no pump is present, obtain sam- ples by attaching a clean bottle to a pole or extension and collecting and mix- ing several samples into a “composite,” which is sent to the laboratory.

    Package and mail all samples to the laboratory as soon as possible to prevent chemical changes in the water during storage. Keep good records of the date and location of each sample. This can best be done by keeping a copy of the Laboratory Information Sheet that must be submitted with each sample.

    In most cases, a Routine Irrigation Water Analysis is the most appropriate test to request for irrigation water. Regardless of the laboratory selected, be cer- tain that the analysis includes the three major factors—total soluble salts, sodi- um hazard (SAR) and individual potentially toxic ions. For special cases or if uncertain, contact your County Extension Office for information.

    For additional information, see our website at http://soilcrop.tamu.edu.

    HOW TO GET A WATER TEST to produce a c

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